GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderMcConnell blocks House bill to reopen government for second time Senators restart shutdown talks — and quickly hit roadblocks GOP senators propose bill to pay 'excepted' workers during shutdown MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonTexas man indicted over allegations he created fraudulent campaign PACs FISA shocker: DOJ official warned Steele dossier was connected to Clinton, might be biased Pompeo’s Cairo speech more ‘back to the future’ than break with past MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchPhRMA CEO 'hopeful' Trump officials will back down on drug pricing move Live coverage: Trump AG pick grilled on Mueller probe at confirmation hearing Trump praises RNC chairwoman after she criticizes her uncle Mitt Romney MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerThe Memo: Romney moves stir worries in Trump World Senate GOP names first female members to Judiciary panel Former US special envoy to anti-ISIS coalition joins Stanford University as lecturer MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellOcasio-Cortez rips Trump in first House floor speech: 'It is not normal to shut down the government when we don’t get what we want' Overnight Health Care: Dem chair plans hearing on Medicare for all | Senate GOP talks drug prices with Trump health chief | PhRMA CEO hopeful Trump reverses course on controversial pricing proposal Supporters leave notes on plaque outside Ocasio-Cortez's office MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerCardi B expresses solidarity with federal workers not getting paid Government shutdown impasse is a leveraging crisis Overnight Health Care: Dem chair meets Trump health chief on drug prices | Trump officials sued over new Kentucky Medicaid work rules | Democrats vow to lift ban on federal funds for abortions MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurrayOvernight Health Care: Dem chair plans hearing on Medicare for all | Senate GOP talks drug prices with Trump health chief | PhRMA CEO hopeful Trump reverses course on controversial pricing proposal Bold, bipartisan action on child care will win plenty of friends GOP seeks health care reboot after 2018 losses MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeyDems blast EPA nominee at confirmation hearing Overnight Energy: Watchdog investigating EPA enforcement numbers | EPA's Wheeler faces Senate grilling | Interior's offshore drilling staff returning to work during shutdown EPA's Wheeler faces grilling over rule rollbacks MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenTrump taps Commerce watchdog to be new Interior inspector general Here are the lawmakers who will forfeit their salaries during the shutdown Christians at risk of extinction in land where Christmas began MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpPentagon update to missile defense doctrine will explore space-base technologies, lasers to counter threats Giuliani: 'I never said there was no collusion' between the Trump campaign and Russia Former congressmen, RNC members appointed to Trump administration roles MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnBarr hearing marks first time Senate Judiciary has GOP women serving on panel Live coverage: Trump AG pick grilled on Mueller probe at confirmation hearing Overnight Defense: Appeals court sides with Trump on transgender military ban | Trump threatens years-long shutdown | Trump floats declaring national emergency to build wall with military MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.